Edge-emitting semiconductor lasers are finding application in many areas of technology, from optical fiber communication systems to compact disc players to read/write optical data storage systems. In many of these applications it would be highly desirable if the cross section of the laser beam were substantially circular. For instance, it will be readily recognized that the attainable coupling efficiency between an edge-emitting laser and an optical fiber (especially a single mode fiber) would typically be much higher if the laser had a substantially symmetric far field pattern than it is if the laser has a highly asymmetric far field pattern. As is well known, conventional edge-emitting semiconductor lasers typically have a highly asymmetric far field pattern.
For instance, S. Uehara et al., Optoelectronics, Vol. 5, pp. 71-80 (1990) describe a 0.98 .mu.m InGaAs strained-layer quantum well laser that is said to be an efficient pumping source for an Er-doped fiber amplifier. As disclosed on page 78 of that publication, the full width at half maximum (FWHM) of the far field pattern of the laser was 7.degree. parallel to the junction of the laser but was 50.degree. perpendicular to the junction, resulting in a coupling efficiency to single mode fiber of only about 20%.
V.S. Shah et al. Journal of Lightwave Technology, Vol 8(9), pp. 1313-1318 (1990) disclose a technique for improving the coupling efficiency between a conventional laser (i.e., a laser having a highly elliptical far field pattern) and a single mode fiber. The technique involves providing a fiber with a wedge-shaped end face and, typically, an up-tapered end. Maximum attained coupling efficiency was 47%. Such a technique clearly would be difficult to implement outside of the laboratory, and in any case applies only to laser/fiber combinations.
Y.C. Chen et al. Electronics Letters, Vol. 26(17), pp. 1348-1350 (1990) (incorporated herein by reference), disclose a laser having a beam divergence in the direction perpendicular to the junction that is comparable to that parallel to the junction. The laser is a single quantum well laser that comprises four passive waveguides, the resulting structure having 5 eigenmodes, with the operating eigenmode said to be the one with the largest confinement factor with the gain medium. If such a laser is operated at high power levels it frequently will not only emit the fundamental mode but also higher order modes, again resulting in decreased coupling efficiency.
Decreased asymmetry of the far field pattern would not only make possible increased light collection efficiency but could, inter alia, also make it possible to relax the requirements of aberration correction. A relatively circular, low divergence-angle beam (e.g., 10.degree. by 10.degree.) also could greatly relax the misalignment tolerance for laser/fiber coupling.
In view of the advantages that could be realized if a single mode edge-emitting semiconductor laser with reduced asymmetry of the far field pattern were available, lasers that can be reliably single mode at all relevant power levels and that can have a substantially symmetric far field pattern would be of great interest. This application discloses such lasers.